If you have a Sonlet Party that is open, but you want to remove the items that have been claimed, go to your Sales page, select the items you want to remove from the Party, scroll down and click Remove Selected from Parties.
It will give you the option of which Party to remove from. Select and confirm.
If you have an item set to load to a Party, but it sells elsewhere before the Party begins, go to your Listings page. Select it, then mark it as Sold Outside of Sonlet. This will move the item to your Sales page. From there, select the item, scroll down and click "Remove Selected from Parties" and select the Party it was set to load to.